US8323593B2 - Process for solubilizing, reducing and fixing hexavalent chromium contained in chromite ore processing residue into trivalent chromium - Google Patents
Process for solubilizing, reducing and fixing hexavalent chromium contained in chromite ore processing residue into trivalent chromium Download PDFInfo
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- US8323593B2 US8323593B2 US13/153,083 US201113153083A US8323593B2 US 8323593 B2 US8323593 B2 US 8323593B2 US 201113153083 A US201113153083 A US 201113153083A US 8323593 B2 US8323593 B2 US 8323593B2
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B09—DISPOSAL OF SOLID WASTE; RECLAMATION OF CONTAMINATED SOIL
- B09C—RECLAMATION OF CONTAMINATED SOIL
- B09C1/00—Reclamation of contaminated soil
- B09C1/08—Reclamation of contaminated soil chemically
Definitions
- COPR Chromite Ore Processing Residue
- Cr(VI) hexavalent chromium
- Lime (CaO) was used as the base
- sodium carbonate was used as a source of both base and sodium ion
- atmospheric oxygen was the oxidant.
- Talm J 2006, Mineralogical properties of chromium ore processing residue and chemical remediation strategies, Ph.D. Thesis ( Civil Env. Eng ) U. Wisc-Madison).
- the waste contained unreacted chromite ore, various alkaline calcium compounds, and other waste material. Some hexavalent chromium was still present, predominantly trapped in calcium compounds in the waste.
- waste Millions of tons of the waste have been used as landfill material in many areas in the Eastern U.S. (predominantly in New Jersey and Maryland) as well as in Europe.
- Such waste is highly alkaline, and it contains hexavalent chromium as well as trivalent chromium.
- Hexavalent chromium leaches out of the waste causing environmental problems. Leaching hexavalent chromium may also render the waste “hazardous” under U.S. EPA regulations.
- the waste generates an alkaline leachate and can expand over time, causing heaving problems.
- Cr(III) Treatment of materials contaminated with Cr(VI) involves reducing hexavalent chromium to the trivalent form (Cr(III)). Cr(III) is insoluble in neutral and moderately basic solutions due to the precipitation of Cr(OH) 3 (or, if iron is present, as a mixed iron-trivalent Cr oxide).
- reducing agents include ferrous or elemental iron (Rai D et al., 1989, Environmental chemistry of chromium, Sci. Total Environ. 86:15-23; Palmer CD et al., 1991, Processes affecting the remediation of chromium-contaminated sites, Environ.
- U.S. Pat. No. 6,578,633 to Yen C Y entitled In-situ process for detoxifying hexavalent chromium in soil and groundwater and U.S. Pat. No. 6,955,501 to Yen C Y entitled In-situ process for detoxifying hexavalent chromium in soil and groundwater report a method for the in-situ treatment of Cr(VI) in soil and water by spreading a reducing agent on top of the contaminated area and adding water to infiltrate the reducing agent into the contaminated zone.
- the reducing agents mentioned are ferrous salts, sulfide salts, sodium thiosulfate and organic reducing agents.
- a further challenge associated with treating COPR is the lack of a reliable analytical tool to assess effective removal of hexavalent chromium from within the COPR matrix.
- the alkaline digestion test (EPA SW846 method 3060A, a/k/a alkaline digestion test) now used by regulatory agencies does not accurately measure Cr(VI) remaining in the COPR since excess reductant present in a solid precipitate on the surface of COPR particles immediately reduces Cr(VI) released from the interior of the COPR particles during the alkaline digestion test. Because the excess reductant masks untreated Cr(VI) in the COPR particles, the alkaline digestion test overestimates the effectiveness of treating Cr(VI) in COPR and underestimates the compositional level of Cr(VI) remaining in the COPR particles. It therefore remains challenging to design effective treatment methods for treating Cr(VI) in COPR.
- the invention is an analytical method for assessing the compositional level of Cr(VI) in a COPR waste matrix.
- the analytical method can be employed, inter alia, as a preliminary lab-scale tool when designing a field-scale COPR waste treatment method, or as an after-the-fact monitor of the effectiveness of a field-scale COPR waste treatment method.
- the analytical method provides information about suitable stoichiometry of the Cr(VI) and the reductants, as well as suitable relative ratios of the reductants. If used as a monitoring process, the analytical method can yield information about the Cr(VI) compositional level of a treated COPR waste that could indicate whether further treatment is warranted.
- COPR waste matrix is treated using first and second reducing agents, of the sort previously described, to release Cr(VI) from the particles into the waste matrix, whereupon some of the Cr(VI) is reduced.
- a COPR waste matrix of interest ground to a convenient size, such as particles that pass through a 0.5 inch sieve
- matrix particles sufficiently small that they contain no residual unreduced Cr(VI) after the treating step are sieved out and discarded.
- Remaining unbound (dissolved) or particulate reducing agents formed in situ including reducing agent compounds related to the first and second reducing agents that can reduce Cr(VI) upon release from the particles, are removed. Then, the remaining particles from which the very small particles and the reducing agents have been removed are tested in a conventional alkaline digestion test to determine an accurate measure of released Cr(VI). This measure, obtained in the absence of available reducing agents, is more accurate than was previously possible using the alkaline digestion test because, Cr(VI) is not immediately reduced upon release from the particles, so the apparent Cr(VI) level in the particles is not artificially low.
- the unbound reducing agents can be removed by washing the treated material.
- the remaining particles can be abraded to remove surface-bound reducing agents (or related compounds) from the particle surfaces.
- the invention is a treatment process for reducing hexavalent chromium (Cr(VI)) trapped in a particulate COPR waste matrix, where the process comprises the sequential steps or acts of exposing the particulate COPR waste matrix to an amount of a first reducing agent (such as Fe(II), optionally in the form of FeSO 4 ) effective to at least partially solubilize (break down) the matrix and effective to at least partially chemically reduce the Cr(VI) to Cr(III), and when the chemical reduction has run its course, exposing the at least partially broken down matrix to a second reducing agent (such as sulfide ions, optionally in the form of sodium bisulfide) in an amount sufficiently high to diffuse into the COPR waste matrix particles and thereby reduce to Cr(III) the residual Cr(VI) trapped in the particles.
- a first reducing agent such as Fe(II), optionally in the form of FeSO 4
- a second reducing agent such as sulfide ions, optionally in the form of sodium bis
- the COPR waste matrix particles treated in the method are sized to permit convenient handling and effective diffusion of the second reducing agent into the particles.
- the reducing agents are provided in stoichiometric excess relative to the amount of Cr(VI) in the particulate waste matrix, which can be determined using the disclosed analytical method.
- the minimal effective total dose of the two reductants should preferably be used.
- the second reducing agent can be added as soon as the first reducing agent has reacted with the waste matrix, typically just a few minutes (e.g., less than about 5 minutes, or less than about 2 minutes).
- an effective total reductant dose is contemplated in a typical case to be at least about a 1.5-fold to about a 2-fold stoichiometric excess of total reductants relative to amount of Cr(VI) in the waste matrix.
- the amount of the second reducing agent is higher than the amount of the first.
- relative stoichiometric ratios of the second and first reductants of about 5-10 to 1 are suitable.
- the treatment would be effective at higher ratios, but cost considerations warrant limiting the amount of the second agent employed.
- somewhat lower ratios can be sufficient to reduce the residual Cr(VI), but will prolong the duration of the treatment.
- the term “about” signifies up to a 5% variation from the stated number.
- a convenient matrix particle size is smaller than 1 inch (i.e., particles can pass through a 1 inch sieve). In some embodiments, the particles can pass through a 0.5 inch sieve. It will be understood that still smaller particles can be more readily treated in the method, but that for reasons of cost and efficiency, it is appropriate to employ particles in this size range or thereabouts.
- the particles for treatment in the method can be sized by crushing ex situ.
- FIG. 1 illustrates compositional Cr(VI) content in sieved (large) particles over time treated with 5% ferrous sulfate heptahydrate and a 4% sodium bisulfide.
- FIG. 2 illustrates compositional Cr(VI) content in sieved (large) particles after 1 day reaction time with 4% sodium bisulfide and varying ferrous sulfate concentrations.
- FIG. 3 illustrates compositional Cr(VI) content in sieved (large) particles after 1 day reaction time with 5% ferrous sulfate heptahydrate and varying sodium bisulfide concentrations.
- the instant invention is directed to a method of advantageously reducing substantially all of the Cr(VI) bound in the COPR waste matrix, by releasing some of the Cr(VI) bound in the COPR waste matrix or by treating substantially all of the unreleased Cr(VI) bound in the COPR waste matrix.
- the invention relates to the inventor's observation that residual Cr(VI) typically remains tightly packed in particulate COPR waste matrix material, even after treatment. While fine particles of very small diameter (e.g, particles that can pass through a 0.25 mm sieve) do not contain trapped Cr(VI), Cr(VI) remains inaccessible to reducing agents in larger particles for treatment or for quantitative analysis using, for example, the alkaline digestion test. As such, the most recalcitrant Cr(VI) is located inside such larger particles. Whether employed in a lab-scale setting in advance of a field-scale treatment or as a monitor of field-scale treatment outcome, the analytical method described herein selects these large unreacted particles for analysis as reliable indicators for overall treatment progression.
- fine particles of very small diameter e.g, particles that can pass through a 0.25 mm sieve
- Cr(VI) remains inaccessible to reducing agents in larger particles for treatment or for quantitative analysis using, for example, the alkaline digestion test.
- the most recalcitrant Cr(VI) is located inside
- Minute amounts of Cr(VI) in the treated material might not be associated with the large particles and as such is not considered in monitoring COPR Cr(VI) remediation.
- the disclosed methods can be used as an indicator that if all of the Cr(VI) in the larger particles has been treated, then the inference is that all of the Cr(VI) in the waste has been treated. It is highly improbable that Cr(VI) could be present in the abraded material after all the Cr(VI) in the interior of the larger particles has been treated.
- the first and second reducing agents employed are Fe(II) and sulfide ions, provided in the form of FeSO 4 and sodium bisulfide, respectively.
- the skilled artisan can readily adapt the disclosure to employ other reducing agents to reduce Cr(VI), such as other sources of ferrous iron or reduced sulfur species.
- the reducing activities of, and interactions between, these agents in COPR waste matrix treatment are known.
- iron sulfide (FeS) forms in situ and precipitates on the surface of waste particles.
- the treated material is washed to remove unbound reducing agents and unbound FeS.
- the material is then passed through a sieve (of about 0.25 mm to about 0.5 mm) to isolate COPR particles which are then subjected to gentle abrasion, e.g., with a metal spatula, to remove the surface layer of FeS and repeated washing with deionized water.
- a sieve of about 0.25 mm to about 0.5 mm
- gentle abrasion e.g., with a metal spatula
- the skilled artisan can readily adapt the disclosure to employ other fine sieve sizes to isolate particles that contain unreacted Cr(VI) from the finer material in which Cr(VI) has been reduced.
- the vast majority of unreacted Cr(VI) is contained within these particles because unprotected surface Cr(VI) is readily accessible to treatment reagents. Consequently, little, if any, residual Cr(VI) is removed during the particle surface abrasion step.
- Effective removal of surface FeS can be monitored visually as the FeS forms a dark black coating on the lighter-colored COPR particles.
- the samples are washed and abraded until the FeS coating has been removed.
- the abraded particles are then dried (e.g., in an oven at 105° C.) and ground to a fine powder.
- the ground, dried samples are further washed with deionized water to remove excess dissolved sulfide ions that may be present in the pores of the solid.
- the water can be heated to accelerate diffusion of the sulfide ions from the ground particles.
- the samples are then analyzed for compositional Cr(VI), for example using the alkaline digestion test (SW846 Method 3060A).
- Table 1 illustrates an example of the effectiveness of this analytical method, compared to conventional methods.
- COPR samples were treated with varying ferrous sulfate (FeSO 4 .7H 2 O) and sodium bisulfide (NaHS) concentrations, as shown.
- Residual Cr(VI) levels were measured either with conventional methods, i.e., without isolation of large particles (“unsieved”), or with the disclosed methods that include separation of large particles.
- Conventional detection methods indicated compositional Cr(VI) values below detection limits ( ⁇ 1 mg/kg) in COPR at all treatment conditions, suggesting complete Cr(VI) remediation from the COPR.
- the disclosed method revealed large amounts of Cr(VI) left behind in the large particles following most of the tested treatment conditions. Cr(VI) values of over 1000 mg/kg were revealed using the disclosed methods, while conventional methods showed no remaining Cr(VI) in the sample.
- the disclosed methods also revealed important trends with regard to both treatment time and dosage. Measuring Cr(VI) content in sieved particle according to the disclosed methods revealed that Cr(VI) content decreased with time for all doses tested, except for the lowest dose which did not contain sufficient reductant to reduce all the Cr(VI) in the sample. An example of such trend is illustrated by FIG. 1 for treatment with 5% ferrous sulfate heptahydrate and a 4% sodium bisulfide. This graphic representation suggests a first order reaction as the results are roughly linear when plotted as log time versus compositional Cr(VI) level.
- Measuring Cr(VI) content in sieved particles according to the disclosed methods also revealed that increasing ferrous iron concentration inhibits, rather than advances, Cr(VI) reduction.
- FIG. 2 illustrates this finding.
- Compositional Cr(VI) value is plotted versus increasing ferrous sulfate concentrations at constant sulfide concentration and treatment time.
- the methods also revealed that increasing sulfide concentrations at a constant ferrous sulfate concentration and treatment time, results in enhanced Cr(VI) remediation, i.e., decreased compositional Cr(VI) content in treated particles ( FIG. 3 ).
- Table 1 Compositional Cr(VI) in unsieved COPR and in isolated large COPR particles, for varying ferrous sulfate and sodium bisulfide doses and after varying reaction times.
- the disclosed methods can be used to quantify residual Cr(VI) in COPR before and after remediation.
- the disclosed methods can also be used to evaluate treatment progression, for example, by taking successive samples throughout repeated treatment.
- the disclosed methods can also be used to assess the reaction kinetics of Cr(VI) reduction for varying treatment parameters.
- the methods are superior to those known in the art because the particle separation steps permits analysis of residual Cr(VI) without masking the untreated Cr(VI) by the treatment reagents present in the fine fraction of the samples. Using these methods to analyze reaction kinetics during treatment has allowed targeted modification of treatment parameters and, thereby, optimization of the treatment process, which had not been possible prior to use of the inventive methods.
- the findings obtained using the disclosed methods are important in devising effective COPR remediation strategies using ferrous iron and sulfide. Because slow reactions occur over time (for days or weeks) inside the larger particles, particle size reduction is important for enhancing the treatment process. Controlling the relative ferrous iron and sulfide ratio further provides a novel treatment method having enhances effectiveness. In-situ treatment processes, such as those taught by Higgins, do not allow for controlling ferrous iron to sulfide ratios because Higgins injects the reagents underground and therefore cannot effectively control treatment of the Cr(VI) in the interior of the particles.
Abstract
Description
TABLE 1 | ||
Cr(VI) in COPR based on Sieved Solids, after time, | ||
Cr(VI) in | mg/kg |
Dose | unsieved | 0.1 |
FeSO4—7H2O | NaHS | COPR, mg/kg | Day. | 1.0. | 3.0 | 7.0/10.0 |
Untreated | 5400 | 30 | 150 |
1.0 | 1.25 | <1 | 393 | 221 | 121 | 115 | 90. | 131 |
2.5 | <1 | 188 | 94 | 90 | 67 | 69 | 42 | |
4.0(a) | <1 | 166 | 71 | 57 | 30 | 17 | <1 | |
4.0(b) | <1 | 75 | 55 | 2 | <1 | 31 | <1 | |
2.5 | 2.0 | <1 | 659 | 371 | 288 | 142 | ||
4.0 | <1 | 538 | 186 | 203 | 51 | |||
6.0 | <1 | 437 | 124 | 133 | <1 | |||
8.0 | <1 | 270 | 106 | 1 | <1 | |||
5.0 | 0.5 | <1 | 946 | 642 | 191 | 112 | 51 | |
1.0 | <1 | 704 | 570 | 190 | 128 | 38 | ||
1.0 | <1 | 875 | 639 | 209 | 119 | 39 | ||
2.0 | <1 | 608 | 534 | 140 | 45 | 9.5 | ||
2.0 | <1 | 552 | 421 | 564 | 216 | |||
4.0 | <1 | 456 | 216 | 149 | 120 | |||
6.0 | <1 | 290 | 159 | 113 | 48 | |||
8.0 | <1 | 240 | 89 | 99 | <1 | |||
7.5 | 1.0 | <1 | 950 | 645 | 156 | 102 | 87.5 | |
10 | 0.5 | <1 | 1130 | 570 | 350 | 289 | 244 | |
1.0 | <1 | 1020 | 500 | 315 | 330 | 194 | ||
2.0 | <1 | 791 | 397 | 160 | 316 | 125 | ||
4.0 | <1 | 56 | 238 | 66 | 146 | 73 | ||
20 | 1.0 | <1 | 847 | 490 | 253 | 333 | 157 | |
2.0 | <1 | 701 | 384 | 198 | 298 | 119 | ||
4.0 | <1 | 522 | 330 | 156 | 261 | 97 | ||
Claims (18)
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US12/628,494 US20100135876A1 (en) | 2008-12-01 | 2009-12-01 | Process for solubilizing, reducing and fixing hexavalent chromium contained in chromite ore processing residue into trivalent chromium |
US13/153,083 US8323593B2 (en) | 2008-12-01 | 2011-06-03 | Process for solubilizing, reducing and fixing hexavalent chromium contained in chromite ore processing residue into trivalent chromium |
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CA2908548A1 (en) * | 2014-10-17 | 2016-04-17 | Redox Technology Group, Llc | Method for treating soil and groundwater containing heavy metals including nickel |
CN106984644A (en) * | 2017-05-10 | 2017-07-28 | 青岛科技大学 | The preparation of calcium polysulfide renovation agent and its application in pollution law |
CN111077090B (en) * | 2019-12-10 | 2020-09-11 | 中国环境科学研究院 | Cr in soil containing large amount of reducing agent6+Is detected by |
CN111077091B (en) * | 2019-12-16 | 2020-10-02 | 中国环境科学研究院 | Method for detecting hexavalent chromium in soil |
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